4.3 -The Equine Body

Thoroughbreds, along with all other equine breeds, have several characteristics that make the horse a unique animal. There are two systems that will be focused on in this section: systems of support and movement, and systems of information and control (Brown and Powell-Smith, 1994).

Support and Movement

The systems of support and movement include the skeletal and muscular system. The horse’s skeleton consists of over 200 bones (Brown and Powell-Smith, 1994) that are jointed in one of three ways. Immovable joints, such as those in the skull, are fused together to prohibit any movement. The vertebrae of the spinal column are joined together in slightly movable joints. Freely movable joints make up the majority of the junctions in the appendicular skeleton (Figure 4.7), which consists of the limb bones. There are several types of freely movable joints: hinge-type joints, such as the fetlock or human elbow, plane-type joints, such as the knee where two surfaces glide over each other, pivot joints which permit turning of the head, and ball-and-socket joints such as the hip. Each freely movable joint is enclosed in a capsule lined with a lubricating synovial membrane (Brown and Powell-Smith, 1994). The skeleton gives shape to the horse’s body, protects the vital organs such as the brain and spinal cord, and produces essential marrow. For the purpose of studying, the horse’s skeleton can be easily separated into two structures, the axial skeleton and the appendicular skeleton (Brown and Powell-Smith, 1994).

Figure 4.6 -Axial Skeleton

Source: Brown and Powell-Smith, 1994.

The axial skeleton, shown above in Figure 4.6, consists of the skull, spine and ribs. The main function of the axial skeleton is to protect the brain, spinal cord and internal organs. The skull contains many small bones that are fused together in immovable joints to protect the brain and sensory nerves. The skull also contains the high-crowned teeth that are characteristic of the modern horse. There are approximately forty-six vertebrae, including an atlas and axis vertebra, eighteen thoracic vertebrae, six lumbar vertebrae, five sacral vertebrae and fifteen to twenty coccygeal vertebrae. The number of vertebrae varies between breeds; Arabians tend to have fewer vertebrae than other breeds. There is a channel through all vertebrae which permits the spinal cord to rest inside. The eighteen pairs of ribs connect directly to the thoracic vertebrae. The first eight pairs are fused to the sternum while the remaining ten pairs are joined to the sternum through cartilage.

Figure 4.7 -Appendicular Skeleton

Source: Brown and Powell-Smith, 1994

The appendicular skeleton (Figure 4.7) reveals a unique anatomical feature in horses. The front legs are not attached to the supporting axial skeleton with joints of any kind. Instead, the shoulder is connected with a “sling” (Brown and Powell-Smith, 1994) made of muscles, tendons and ligaments which gives the horse a built-in shock-absorption system. This shock-absorption system works best when the slope of the shoulder and fetlock is parallel, as shown in Figure 4.8. The hoof completes the shock-absorption, since the bones of the hoof are attached to the internal hoof way with only sponge-like laminae. The front limbs of the horse carry as much as 60% of the horse’s body weight and take the brunt of the impact from jumping and running; a complex shock-absorption system is therefore necessary.

Figure 4.8

Source: Meadows, 1999

From the knee down, the horse’s legs contain bones that are comparable to human hands (Biracree and Insinger, 1982). The third metacarpal (the bone that runs from wrist to knuckle) simulates a horse’s cannon bone. The knuckle itself is the horse’s fetlock jointand the three segments of our fingers correspond to the pasterns and coffin bone. See Figure 4.9 for a pictorial analysis of this comparison.

Figure 4.9 –Comparison of Human and Equine Skeleton

Source: Brown and Powell-Smith, 1994

The pelvis, or hip joint, which is a complete hoop of bones protecting the vital reproductive systems, is made up of three bones, the ilium, the ischium and the pubis (Brown and Powell-Smith, 1994). The hind limbs also compare to human anatomy. At the distal end of the femur, above the stifle joint, is a patella or kneecap. Although a tibia and fibula are present in both humans and horses, the equine fibula has been reduced to a vestigial remnant over the millions of years of evolution. The hock corresponds to the human ankle. Below the hock, the bones of the lower hind legs are similar to the lower forelimbs.

Another mechanical advantage that can be seen in Figure 4.10 is the lack of major muscle groups in the lower legs of the horse. In place of muscles, the horse’s lower leg is equipped with a group of tendons that connect the bones of the lower leg to muscle pairs of the upper leg, shoulder and hip. This permits “reciprocal movement” of the lower leg. The horse has to work only the muscles in the shoulder and upper leg in order to move the lower leg.

Figure 4.10 -Muscular System

Source: Brown and Powell-Smith, 1994

A muscle of interest to halter class participants is the trapezius, which lies over the withers. Halter classes are separate from the riding classes at horse shows where saddle-free horses are hand-walked in front of a judge, who determines which horse has the best overall confirmation. When a horse’s confirmation, which is the overall shape of various body parts, is being judged in a halter class, well-developed trapezius muscles make the withers and neck flow together. Under-developed trapezius muscles make the withers stand too high, giving the appearance of an uncomfortable ride. The brachiocephalicus muscle is responsible for pulling the shoulder forward during work. It is important to recognize the position and function of this muscle in certain sports. For example, jumpers should be given extra rein so that the forward head carriage will allow the brachiocephalicus muscle to work properly and pull the horse over the jump, while horses performing collected paces should carry their heads high to pull the shoulder up instead of forward.

Information and Control

The primary mechanism of information and control is the sensory system, consisting of sight, hearing, smell, taste and touch (Brown and Powell-Smith, 1994). The sensory organs have evolved over the Cenozoic era to provide the horse with the best information about its habitat. As Miohippus made the transition from browsing in forests to grazing in open grasslands, the eyes became larger and positioned further back on the sides of the head. The laterally placed eyes give the horse a 215° monocular field of vision on the sides and back and a 70° binocular field of vision in front (see Figure 4.11). The horse had two blind spots, directly in front of their face and directly behind their bodies. The 285° field of vision allows a horse to see what is on the ground under them while grazing, while not compromising the vision of the open grassland where predators may be hiding (Brown and Powell-Smith, 1994). The horse can see movement in the far distance better than humans. However, stationary objects in the far distance are not normally seen by a grazing horse. The debate over the color range horses are capable of visualizing is still on-going. Color detection tests so far suggest that horses see yellow and green hues better than any other colors (Waring, 1982).

Figure 4.11 –Equine Field of Vision

Source: Brown and Powell-Smith, 1994

The equine sense of hearing is also very specialized. Horses are capable of moving each ear independently in response to sounds without the need for moving their head. While the majority of equine sound perception is within the limits of human comprehension, horses are able to hear low frequency geographical vibrations such as those preceding earthquakes. They can also hear high frequency sounds above human perception, although this ability declines as the horse ages (Waring, 1982).

Smell and taste are received through three sensory receptor organs housed within the horse’s head (Brown and Powell-Smith, 1994). The olfactory nerve endings in the mucous membranes of the nasal cavity are covered with millions of tiny hairs that detect smells. The vomeronasal organ lies beneath the floor of the nasal cavity. It is used in flehman response behavior (Figure 4.12) typically associated with stallions. Scientists suspect the stallion fills the nasal cavity with odor laden air and closes the nasal passage by raising the upper lip. The odor laden air then flows into the vomeronasal organ as the horse’s head is raised where it can be analyzed for chemical or hormonal content (Waring, 1982). The taste buds are responsible for the sense of taste. Most taste buds are found on the tongue, although they are also present on the palate and in the throat (Brown and Powell-Smith, 1994). Equine taste buds can detect salty, sweet, bitter and sour tastes. Horses seem to prefer salty and sweet tastes, avoiding bitterness and sourness whenever possible (Brown and Powell-Smith, 1994). Taste is not easy to analyze in animals. However, tests done during the 1970’s showed foals had a preference for sucrose/water solutions, were indifferent to salty mixtures and refused sour and bitter preparations (Waring, 1982). The selective taste buds protect wild and domestic horses from ingesting bitter tasting poisonous plants.

Figure 4.12-Flehman

Source: www.seaworld.org

The manner in which horses perceive touch can be divided into five groups: touch, pressure, heat, cold and pain (Brown and Powell-Smith, 1994). Horse skin has a special ability to “twitch” in reaction to flies or other pests touching the body. Pressure can be used to human advantage, such as when applying a humane twitch(Figure 4.13) to a horse’s sensitive muzzle. The twitch activates the numerous pressure sensors in the muzzle, calming the horse so that a veterinarian or farrier can work on a different area of the body, minimizing pain reactions; twitches are often used when breeding mares. The skin also has the ability to cool itself by sweating in hot weather and shivering to produce body heat in cold weather (Brown and Powell-Smith, 1994).